In recent years, along with the advances in computer image treatment technology, a method has been developed for directly drawing an image on a photosensitive layer by emission of light corresponding to a digital signal. Attention is now focusing on computer-to-plate (CTP) systems utilizing that method for a planographic printing plate master and directly forming an image on a photosensitive planographic printing plate without output to a silver salt mask film.
As a negative type photosensitive planographic printing plate for a CTP system, for example, there is the so-called photopolymer type comprising a photosensitive layer made of mainly a photocuring resin formed on a substrate. In a photopolymer type photosensitive planographic printing plate, the photopolymerization initiator included in the photosensitive layer is activated at the time of exposure whereupon radicals are produced and, due to this, the photocuring resin is cross-linked and insolubilized and thereby an image is formed.
However, as explained above, the photopolymer type photosensitive planographic printing plate deteriorates in the bond strength between the photosensitive layer and substrate, so, as described in Japanese Patent Publication (A) No. 2003-57831, it has been proposed to provide an undercoat layer including a polyvinyl phosphoric acid between the photosensitive layer and substrate.
However, even if using such an undercoat layer, it was difficult to sufficiently secure bondability between the photosensitive layer and substrate.